The missing colour in the #a00 to #000 interpolation is because HCL is trying to create a colour outside of sRGB (the green component is less than zero). As for the hue change, one difficulty of making a gradient to black in HCL is that black doesn't have a hue when specified as rgb(0, 0, 0), which seems to mess up some tools.

Edit: It's interesting to see how the different colour spaces react to the #000 being changed to #010000 or #000100 or #000001.